The coal for coking is preheated in many cases before it is charged into the ovens. This can be done for several reasons. There are various kinds of coal which do not yield sufficiently firm coke on degassing but which can provide coke of higher quality if they are charged after preheating. Another reason for preheating the coke is due to the preheated coke being carbonized in a shorter time. This means that a larger quantity of coke can be obtained from the same number of ovens. The capacity of a coking plant is, therefore, greater if the coal is preheated.
Charging preheated coal into the ovens encounters difficulties because the resulting gas contains substantial quantities of dust during charging and for some time thereafter. If these quantities of dust remain in the gas they militate against the further processing thereof. A second gas-collecting main has, therefore, been used in the past for drawing off the gases which are produced when the coal is charged. The operation of such a separate gas-collecting main, however, is difficult in view of the tarry constituents contained in the gas.